Loops

We finished up Chapter 6.1 in class today. We learned that we could use loops in order to accomplish tasks that should be done several times using Loops. We learned about three types of loop structures:

Do Until
Do While
For Next

We can write a loop using any of these three structures and any loop written in one of these structures can be rewritten using one of the other two. The basic syntax for each of these loops are:

For variable = (starting point) To (ending point)
    'Do statements until we reach the ending point
Next 'Update condition variable

'**************************************

'Set condition before entering loop
Do While condition
    'Do statements until condition is false
    'Update the condition
Loop

'**************************************

'Set condition before entering loop
Do  
    'Do statements until condition is true
    'Update the condition
Loop Until condition

In all three loop structures we have three parts that are the same:
1) A starting point for the condition we are checking
2) An endpoint for the condition
3) An update for the condition

We must have all three things for the loop to execute and end execution. Without (3) from above for example, we would have an infinite loop which is definitely not what was intended.

Also notice that the Do While and the Do Until loops are opposites. Do While loops until the condition is False whereas the Do Until loop will loop until the condition is True. Also the Do Until is always guaranteed to loop at least once. We have no such guarantee with the Do While loop.

Do While Examples

Dim i As Integer = 1
Do While i <= 10 
    'List out the numbers from 1 to 10
    lstOut.Items.Add( i )
    i += 1
Loop

Dim i As Integer = 1
Dim num As Integer
num = CInt(InputBox("Enter a #:",""))
Do While i <= num
    'List out the numbers from 1 to num
    lstOut.Items.Add( i )
    i += 1
Loop

Do Until Examples

Dim i As Integer = 1
Do  
    'List out the numbers from 1 to 10
    lstOut.Items.Add( i )
    i += 1
Loop Until i > 10

Dim i As Integer = 1
Dim num As Integer
num = CInt(InputBox("Enter a #:",""))
Do 
    'List out the numbers from 1 to num
    lstOut.Items.Add( i )
    i += 1
Loop Until i > num

For Next Examples

For( i As Integer =  1 To 10)
    'List out the numbers from 1 to 10
    lstOut.Items.Add( i )
Next

Dim num As Integer
num = CInt(InputBox("Enter a #:",""))
For( i As Integer =  1 To num)
    'List out the numbers from 1 to num
    lstOut.Items.Add( i )
Next